1. Field of The Invention
This invention relates to a method of permitting an earth station(s) to initially access its assigned time slot in a TDMA (Time Division Multiple Access) system and also an arrangement therefor, and more specifically to such a method and arrangement featuring that only one or two of the earth stations are required to have an acquisition control arrangement in the overall TDMA system. This invention utilizes a low power burst acquisition signal for the initial access to the satellite.
2. Description of the Prior Art
In the TDMA system, multiple earth stations (viz., reference station and traffic terminals) share one satellite transponder on a time division basis. Each earth station transmits bursts in a manner that each burst is located within an allocated time slot of each consecutive TDMA frame. The burst therefore has the same period as the TDMA frame, and the times of the bursts are carefully controlled using a reference burst transmitted from the reference station so that no two bursts overlap.
FIG. 1 is a diagram showing a TDMA frame format, in which R denotes a reference burst, each of C, C' and C" denotes a burst which is transmitted from an earth station already in communication with the satellite, and D denotes a time slot preassigned to a given earth station but which is not being used.
When an earth station wants to initially access the satellite, the burst transmit timing is unknown to the earth station and hence burst acquisition support is necessary prior to burst synchronization control. To this end there have been proposed several techniques such as a prediction method, low power method using a low level acquisition signal, etc.
The prediction method, disclosed in Japanese patent application laid open under publication No. 42-6417, predicts an assigned time slot according to a computed saltellite orbit. This method, however, has encountered the problem that any error in the prediction might cause the station's burst to overlap other time slots and therefore interupt communications already in progress between other stations. On the other hand, the low power method found to be useful due to its high efficiency in band utilization as well as its simplicity. As a consequence, this invention utilizes a low power acquisition signal.
FIGS. 2(A) and 2(B) are diagrams describing known methods which utilize the low power signals, wherein FIG. 2(A) shows a continuous low power signal E and FIG. 2(B) a low power pulses F (only one is shown). In both figures, the TDMA frame is identical to that of FIG. 1.
The acquisition signal E (FIG. 2(A)) is produced by PSK (Phase Shift Keying) modulating a PN (Pseudo random Noise) sequence or a special digital sequence, and has a lower power than a normal level by approximately 20 dB, for example. The low amplitude acquisition signal E, sent out from the earth station intending to enter communication, is relayed back via a satellite to the same earth station and is demodulated therein. The demodulated acquisition signal E is compared with the sync signal to detect their relative phase difference, thereby allowing an accurate burst transmit timing to be obtained.
On the other hand, with the other low power method shown in FIG. 2(B), the earth station, desiring communication with the satellite, sends out the low power acquisition pulses F and receives same via the satellite. The accurate transmit timing is detected by scanning either automatically or manually the received pulses in order to locate the pulse in the time slot assigned to the earth station.
However, with these methods, all the earth stations (viz., traffic terminals) of the prior art (FIGS. 2(A) and 2(B)) are required to individually perform the initial accessing, and thus requires that each earth station has its own burst acquisition control arrangement. Therefore, these methods suffer from the drawback that each earth station is bulky, complicated in arrangement and expensive to manufacture. Further, these methods should overcome noise interference resulting from the use of the low level acquisition signals. One approach to solving this problem is to use narrow band filtering. Another is to improve a receiving error rate by employing a decision by majority method. These requirements further complicate and increase the cost of each station.